This invention relates to cellular telephone communications and in particular to an apparatus for testing cellular telephone communications equipment.
With the increasing popularity of wireless communication, the resulting infrastructure now includes a vast network of cellular base stations. Wireless communication typically involves the transmission of information between a mobile station such as a handset, and a cellular base station that is connected to a public switched telephone network (PSTN) such as a local or long distance carrier.
During the progress of a call between the mobile station and cellular base station, several types of communications occur. The cellular base station first uses control channels to set up the call to a mobile station, assigning a frequency or channel number, power level, and so on, to the mobile station. Once the communication link has been established, traffic channels are used to communicate voice and data signals between the mobile station and the cellular base station. The cellular base station is in turn connected to the PSTN through a base station controller and mobile switching center.
Each cellular base station consists of a transceiver that is controlled by the base station controller via a fixed link. The base station controller may control a number of other transceivers via separate fixed links. Depending on the cellular standard and protocol, including for example, advanced mobile phone service (AMPS) according to the Electronic Industries Association (EIA) and TDMA (time division multiple access) digital cellular which complies with the IS-54 standard, the radio frequency (RF) signals produced by the cellular base station in response to command received over the fixed link will differ.
A variety of methods have evolved for supporting, testing, and maintaining the increasingly vast network of cellular base stations. The HP 8921/8935 RF Communications Test Set manufactured by Hewlett-Packard Company provides a variety of parametric tests of mobile stations such as cellular telephone handsets as well as cellular base stations. Used in conjunction with various cellular adapters which adapt the HP 8921/8935 for the various cellular standards, the HP 8921/8935 allows for testing call processing, transmitter parameters such as frequency error and output power, and receiver parameters such as audio distortion and frequency response. In some cases, the HP 8921/8935 simulates a mobile station to generate test signals for the cellular base station. However, the HP 8921/8935 does not monitor the fixed link between the cellular base station and base station controller.
Another approach to testing cellular base stations is described in U.S. Pat. No. 5,570,373, "Method and Apparatus for Testing a Radio in a Base Station Without Using a Radio Test Unit", issued Oct. 29, 1996, to Thomas E. Wing. Wing teaches a method for testing the base station in which data about call processing activities are collected, processed and then compared to expected results to detect a failure condition. The method taught by Wing is non-invasive in that no special test signals are generated during the data collection process. However, no actual measurements of the RF signal generated by the cellular base station are made. Instead, a statistical analysis of the collected call processing data is performed to infer the behavior of the cellular base station and detect errors after the fact.
It is desirable to measure the RF behavior of the cellular base station at the antenna in real-time responsive to commands received on the fixed link. Such "live" measurements are often needed when the behavior of the cellular base station is being verified by a service technician. For example, a parametric transmitter measurement that evaluates how quickly the cellular base station can change output power levels responsive to a command from the base station controller must be performed in real-time.
Other approaches for testing cellular base stations include initiating tests in the base station controller which injects test signals into the fixed link to control the cellular base station. The behavior of the cellular base station may then be evaluated using remote monitoring equipment. In U.S. Pat. No. 5,134,643, "Method and System for Testing Mobile Communication Systems", issued Jul. 28, 1992, to Kiyoshi Iwata, a cellular automated test set provides for a line control unit which transmits a line test instruction to a pseudo mobile station and monitors the delivery of the test instruction signal back through the base station. The line control unit judges whether the line connection is normal or abnormal depending on whether or not the station identifying information is received. In U.S. Pat. No. 5,471,649, "Base Station Transceiver Diagnostic Equipment", issued Nov. 28, 1995, to Rees et al., a remote base station diagnostic subsystem operates as a mobile transceiver which is in RF communication with the cellular base station. The remote base station diagnostic subsystem is under the control of the base station controller via a local area network to carry out the tests by the base station controller.
In the test methods taught by Iwata and by Rees et al., the base station controller initiates and runs the tests of the cellular base station. Conducting these tests in an invasive manner requires that the cellular base station be taken out of service for the test and operated in a test mode. Both methods further require additional hardware within the base station controller for storing and executing the test routines and as well as a communication bus separate from the fixed link which allows for control of the test. Supporting an appropriate set of tests in the base station controller that are compatible both with the particular cellular base station and with the external test hardware is problematic. The base station controller might not be physically located at the same cell site as the cellular base station, further complicating testing efforts. Furthermore, neither method taught by Iwata or by Rees et al. teaches making parametric tests of the cellular base station as it responds to commands received from the fixed link. It would therefore be desirable to have a method for testing the cellular base station non-invasively in real-time and without requiring specialized tests to be supported in the base station controller.